Solar cell with reduced shading and method of producing the same

ABSTRACT

A solar cell is disclosed having electrical connections only on the rear side to reduce shading and improve efficiency of the cell. The solar cell includes a crystalline silicon substrate exhibiting crystallographic planes on the front and rear sides and a flat, doped emitter layer on at least the front side. The solar cell also includes a plurality of elongated slots aligned parallel to crystallographic planes and extending through the entire thickness of the silicon substrate of the solar cell. A high doping, corresponding to the conductivity type of the emitter is located in the slots. Two contact patterns are located on the rear side of the solar cell. The first is for electrical connection to the bulk material and the second is for electrical connection to the emitter and at least partly overlaps the slots. The slots are crystallographically etched anisotropically from the rear side and taper toward the front side of the cell. Also disclosed is a method for producing a solar cell. In this method, a plurality of slots are etched into, and extending through the entire thickness of, a crystalline silicon substrate in an alkaline, crystal-oriented and masked manner. A flat emitter layer is produced by diffusion of a dopant. Finally a first and second contact pattern are produced on the rear side of the solar cell by imprinting and burning in a conductive path. The second contact pattern is formed to overlap the slots.

Reduced shading can, for example, be achieved in a solar cell in whichboth the n and the p contacts are located on the rear side. In this waythe front side is not shaded by any contact and is therefore availablewithout restriction for the irradiation of light.

A solar cell without front-side metallization is known, for example,from R. A. Sinton, P. J. Verlinden, R. A. Crane, R. M. Swanson, C.Tilford, J. Perkins and K. Garrison, "Large-Area 21% Efficient Si SolarCells", Proc. of the 23rd IEEE Photovoltaic Specialists Conference,Louisville, 1993, pages 157 to 161. To produce same, varyingly dopedareas are generated side by side in a plurality of masking steps and aremetallized or contacted by applying a multilayer metal structure on topthereof. The metal structures are applied by thin-film techniques.

One drawback is that the method needs a plurality of masking steps andis complex as a result. All the charge carriers also have to reach therear side of the solar cell by way of diffusion, there being a greaterprobability of charge carrier recombination which in turn reduces thesolar cell's collection efficiency.

Another idea for a solar cell without front-side metallization is knownfrom the article entitled "Emitter Wrap-Through Solar Cell" by James M.Gee et al. in a paper for the 23rd Photovoltaic Specialists Conference1993, Louisville, pages 265 to 270. The solar cell described therecomprises an emitter layer placed close to the front side with a pnjunction adjacent thereto. Contact holes drilled and metallized by meansof a laser connect the emitter layer to metallized contacts positionedon the rear side. The rear-side contacts are also disposed on the rearside interdigital to the "front-side contacts". This solar cell suffersfrom the disadvantage of a high number of contact holes that have to bedrilled with a laser, this large number requiring about 10,000 contactholes per solar cell for a typical solar cell 100 cm² in size and atypical gap of 1 mm between the contact holes. This reduces thethroughput in automated production. Furthermore, the contact holes andthe associated contacts disposed on the rear side have to be adjusted inrelation to one another. Undesired structural transformations in thesilicon may also be produced in the contact holes drilled with a laser,thereby making it possible to create additional recombination centersfor pairs of charge carriers which further reduce collection efficiency.The reduced mechanical strength may lead to rupture in these solarcells.

The present invention's object is to design a solar cell withoutfront-side contacts which create shade; such a solar cell is simple andinexpensive to produce and satisfies other requirements for ahigh-output solar cell.

In accordance with the invention, this object is solved by a solar cellaccording to claim 1. Preferred embodiments of the invention and amethod of producing same can be gathered from further claims.

The solar cell according to the invention is built up from a(110)-orientation crystalline silicon substrate. This material enjoysthe advantage that it exhibits (111) planes aligned vertical to the(110) surface. Anisotropic etching oriented toward the crystal structuremakes it possible to generate depressions, holes or openings with a highaspect ratio and two vertical side walls in the (110) substrate. Thesolar cell according to the invention has a plurality of elongated slotsaligned parallel to (111) planes and extending through the entirethickness of the silicon substrate or breaking through this substrate.The inner surfaces of the slots have a high doping corresponding to theconductivity type of the flat emitter layer generated at least on thefront side. A grid-like first contact pattern is located on the rearside of the solar cell for electrically connecting the bulk material.Interdigital thereto, a second grid-like contact pattern which overlapswith the slots at least in part and thus ensures the emitter layer'selectrical connection is disposed on the rear side.

The front side of the solar cell according to the invention isunimpaired except for the slots and has a high-grade surface whichenables good passivation and an effective antireflection layer. Becauseof the good anisotropic etchability in (110)-oriented silicon, the slotscan be generated with high aspect ratios of e.g. 1:600 in the siliconsubstrate. This makes it possible to minimize the size of the slots andhence the surface losses. Slots that have been anisotropically etched in(110) silicon have side walls which consist of (111) planes. Two ofthese planes are disposed vertical to the substrate surface, whereas thetwo "narrow sides" extend at an angle through the substrate. Whenetching from the rear side of the silicon substrate, the cross sectionof the slots therefore tapers toward the front side, so that as aresult, the surface losses are further reduced by the slots on the frontside. The elongated extension of the slots makes it easier to adjust thesecond contact pattern which overlaps the slots on the rear side.

The silicon substrate is highly doped in the slots. This creates currentpaths of electrically sufficient conductivity which connect the frontside of the solar cell to the rear side, or to the contact patternapplied there. A sufficiently dense pattern of slots and the relativelylow substrate thickness cause the current paths to remain short forcharge carriers collected on the front side. In this way, the solarcell's series resistance is also low and a high fill factor is madepossible.

In an advantageous embodiment of the invention, a so-called tricrystalwafer is used as a substrate, as is known for example from an article byG. Martinelli in Solid State Phenomena Vol. 32 to 33, 1993, pp. 21-26.Such a wafer comprises three monocrystalline regions that are tiltedtoward one another and which in themselves are each (110)-oriented. Theboundary areas between the monocrystalline regions extend radiallytoward the middle of the wafer so that the monocrystalline regions formsectors of the tricrystal wafer. Two of the three boundary areas arefirst-order twin grain boundaries on (111) planes which are particularlylow in imperfections.

A solar cell according to the invention produced from such a tricrystalwafer enjoys the advantage that the mechanical stability of the waferand hence of the solar cell is substantially increased compared to amonocrystalline substrate. In this way the substrate thickness can bereduced to values of 30 to 70 μm without having to take an increasedrisk of rupture into consideration during processing. The tricrystalwafer is particularly suitable for the invention because it only has(110)-oriented surfaces or makes (110)-oriented silicon substratessufficiently available for the first time. Crystal pulling ofmonocrystalline (110)-oriented rods is much more difficult than that ofconventional (100) silicon rods, since crystal rearrangements andstructural loss are produced more quickly, such loss causing the pullingprocess to be stopped too early. Crystal pulling of a tricrystal, on theother hand, is 2 to 3 times faster than is the case with (110)-orientedsilicon rods. A cone is not necessary at the end of the rod. It cantherefore be performed quasi-continuously and without rearrangement. Acrucible can be used up to ten times.

A solar cell having a thinner silicon substrate enjoys other technicaladvantages in addition to the saving in material. Using a thinnersubstrate, the demand placed on a high-output solar cell that thediffusion length of the minority charge carriers should be greater thanthe three-fold thickness of the substrate is already satisfied by amaterial of a lower electronic quality. A thinner silicon substrate in asolar cell therefore results in lower recombination losses than athicker substrate.

A solar cell with a tricrystalline silicon substrate is sufficientlystable even when there is a plurality of slots breaking through thesubstrate. It is nevertheless advantageous for the slots extendingparallel to (111) planes to be offset against one another so thatseveral slots which might assist a rupture of the substrate parallel tothe crystal planes as a result of the predetermined "perforation" arenot arranged in succession into one and the same (111) plane.

A first and a second contact pattern on the rear side of the solar cellare preferably applied as thick-film contacts and particularly asconductive pastes to be sinter-fused. The first and second contactpatterns form an interdigital structure in which finger-like contactsare alternately arranged, the finger-like contacts of the first andsecond contact patterns engaging with one another like the teeth of azip fastener. Each contact pattern comprises at least one bus structurewhich connects all the finger-like contacts together. One of the busstructures is preferably arranged circumferentially close to the edge ofthe solar cell's rear side. The surface-area proportions of the firstand second contact patterns are preferably approximately equal becauseidentical charge quantities have to be transported for both chargecarrier types, thus minimizing the series resistance.

The method of producing the solar cell according to the invention willnow be explained in more detail on the basis of exemplary embodimentsand the associated ten figures. The figures belong solely to theexemplary embodiments and should not be regarded as restrictive.

FIGS. 1 to 7 use diagrammatic cross sections through the structure toshow different procedural stages in the production of the solar cell;

FIGS. 8 and 9 use diagrammatic cross sections through the substrate toshow different procedural stages of a process version;

FIG. 10 shows a slot in a perspective horizontal projection of a siliconsubstrate;

FIG. 11 shows a tricrystal wafer in horizontal projection;

FIG. 12 shows a possible configuration for the first and second contactpatterns on the rear side.

The starting point for the process according to the invention is an e.g.p-doped (110)-orientation silicon wafer 1. The slots or a pattern ofslots are produced in the first step. For this purpose, an oxide ornitride layer 2 is first applied all over the entire surface area of thefront side VS and the rear side RS. Rectangular openings 3 thatcorrespond to the slot pattern are then photolithographically definedand freely etched in this oxide or nitride layer 2. FIG. 1 illustratesthis procedural step on the basis of a diagrammatic cross sectionthrough a silicon substrate; this cross section is not true to scale.

In accordance with the pattern of openings 3 defined in the maskinglayer 2, slots 4 are now produced in the substrate 1 by means ofcrystal-oriented alkaline etching. FIG. 2 illustrates this state afterremoval of the masking layer 2.

FIG. 3: a flat, n⁺ -doped emitter layer 5, e.g. at a depth of 0.3 to 2μm, is produced on all the surfaces of the silicon substrate 1,including the slots, as a result of the phosphorus doping that takesplace all over.

FIG. 4: a passivation layer 6, e.g. an oxide or nitride layer which isusually 70 nm thick, is applied all over each surface in the next step.

FIG. 5: the electrical contacts are applied to the rear side in athick-film technique in the next step. As regards the first contactpattern 7, finger-like contacts, in addition to the slots 4, are forexample applied to the rear side RS in order to contact the bulkmaterial, i.e. to contact the inner p-doped substrate region. This maybe brought about for example by imprinting a conductive screen-printingpaste that contains silver or aluminum particles and which can besintered. The paste contains either aluminum or another dopant thatgenerates p doping, such as boron. A second contact pattern 8 is appliedover the slots 4 at least in part, e.g. by imprinting asilver-containing conductive paste. The first and second contactpatterns 7 and 8 are grid-shaped and each comprise at least one busstructure and finger-like contacts emanating therefrom. The two contactpatterns are arranged on the rear side of the substrate such that thefinger-like contacts interdigitally engage with one another and arespatially separate from one another. FIG. 5 shows the configurationafter this procedural step.

FIG. 6: in the next step, the contacts are burned in and sintered, thepassivation layer 6 beneath the contact patterns 7 and 8 being alloyedin an electrically conducting manner. The dopant contained in the pastefor the first contact pattern 7 generates a p⁺ doping 9 whichovercompensates the emitter layer 5 and produces the ohmic contact withthe internally positioned p-doped region of the substrate 1. Thematerial of the second contact pattern 8 produces a conductiveconnection to the n⁺ -doped region 5, the emitter layer.

FIG. 7: in the next step, the first and second contact patterns 7 and 8can be used as a self-adjusting mask to optionally separate the pnjunction between the first and second contact patterns, e.g. by plasmaetching, depressions 13 being produced between the first and secondcontact patterns. If the p+ doping 9, which simultaneously represents aback surface field (BSF), prevents a conductive connection between thecontact pattern 7 and the emitter layer, plasma etching is not, however,necessary.

FIG. 8: in one version of the process (following on from the proceduralstep according to FIG. 4), the passivation layer 6 and emitter layer 5are removed in a lift-off technique, e.g. by brief plasma etching, in aregion 14 which is provided to receive the first contact pattern. Thisregion is therefore somewhat larger in dimension than the first contactpattern.

FIG. 9: the first and second contact patterns 7, 8 are then applied e.g.by means of imprinting and are optionally burned in. The first contactpattern may in turn contain a doping suitable for generation of a BSF.

Following on from the state illustrated in FIG. 4, however, it is alsopossible to first apply the second contact pattern 8 and to use it as amask for the lift-off technique in order to remove the passivation layer6 and emitter layer 5, whereby recesses that correspond to the regions14 and reach right into the bulk material are produced. The firstcontact pattern 7 is then applied in these recesses. In this version, itis advantageous for the second contact pattern 8 to be generated with alarger surface area than the first contact pattern 7 in order to keep amaximum emitter surface after the lift-off process.

In any case, the first and second contact patterns 7 and 8 are appliedsuch that the two do not overlap and are electrically separate from oneanother.

FIG. 10 depicts in a perspective illustration the rear side of thesilicon substrate 1 with one of the slots 4. This slot comprises twoopposite vertical walls 11 which correspond to (111) planes in thesubstrate. The narrow sides of the slots 4, on the other hand, aredelimited by crystal faces 12 extending at an angle thereto and whichalso represent (111) planes. When defining the slot pattern in themasking layer 2 at the start of the process, it is borne in mind thatthe longitudinal axis of the slots is disposed parallel to the vertical(111) planes. Length 1 and width b of the slots (on the rear side) arechosen such that an opening which breaks through the substrate 1 is justproduced during crystal-oriented etching. The slot width b is set to 5to 50 μm and varies for example from 15 to 20 μm. The slot length ldepends on the thickness of the silicon substrate 1. The length l ispreferably chosen such that the virtual intersection of the faces 12which delimit the narrow sides of the slot is disposed just above thefront side VS of the silicon substrate 1. This produces a slot which isexamined from the front side VS of the substrate 1 and whose "length"corresponds to b and whose "width" is minimized parallel to the slotlength l.

FIG. 11 shows a tricrystal wafer which is preferably used as a substratefor the solar cell according to the invention. This wafer comprisesthree monocrystalline regions M1, M2 and M3 which are all (110)oriented, but tilted toward one another.

In the figure, the tricrystal wafer is disposed such that a first-ordertwin grain boundary KG12 having (111) planes as crystal faces thatdelimit the grain is produced between the monocrystalline regions M1 andM2. The grain boundary KG13 between M1 and M3 is also a first-order twingrain boundary with delimiting (111) crystal planes. An optimally growntricrystal having the two first-order twin grain boundaries has idealinternal angles between the different monocrystalline regions whichamount to exactly 109.47° for W1 and exactly 125.26° for W2 and W3.Internal angles which deviate therefrom also result in a stabletricrystal wafer which can be obtained by sawing it out of correspondingtricrystal rods, whereby reliable handling of the corresponding wafer isguaranteed down to wafer thicknesses of 30 μm without any increased riskof rupture. Wafer thicknesses that are preferred for a solar cell rangefor example from 60 to 150 μm.

FIG. 11 shows an exemplary embodiment for the configuration of first andsecond contact patterns on the rear side of a tricrystal wafer. Inaccordance with the orientation illustrated in FIG. 9, the two lowershanks of the "star" formed by the grain boundaries form first-ordertwin grain boundaries. The slots in the tricrystal wafer are preferablydisposed such that their length l is aligned parallel to one of thefirst-order twin grain boundaries. The slots are preferably alignedparallel to that first-order twin grain boundary which is closest to theslot. Corresponding to the configuration of the tricrystal waferdepicted in FIG. 9, the slot pattern is aligned parallel to the grainboundary KG13 in a first wafer half on the left of the imaginary axis A,but aligned parallel to the grain boundary KG12 in the wafer half on theright of the axis A. The slots are preferably offset against one anotherso that slots arranged side by side in a row do not end up in one andthe same (111) plane. They are preferably offset against one another bymore than a whole slot width.

A second contact pattern 8 suitable therefor and overlapping all theslots is illustrated for example in FIG. 12. The first contact pattern 7has a bus structure which is arranged circumferentially close to thesubstrate edge. Contact fingers emanating therefrom point at an angle tothe substrate's central axis. The second contact pattern 8, on the otherhand, has a central bus structure which is for example disposed parallelto the axis A shown in FIG. 9. The finger-like contacts emanatingtherefrom are disposed interdigital to the first contact structure 7without touching same. The geometrical alignment of the first contactpattern 7 [should read: 7] is chosen in the exemplary embodiment suchthat the contact fingers are aligned parallel to the length l of theslots and they therefore overlap in terms of length. The first contactpattern 7 does not overlap any of the slots. But it is also possible tochange around the assignment of the contact patterns to the p andn-doped areas of the solar cell so that for example the contact patternwith the circumferential bus structure overlaps the slots and thereforecontacts the n-doped areas, whereas the contact pattern with the centralbus structure serves to contact the p-doped bulk material.

The width of the finger-like contacts for the first and second contactpatterns is set for example to about 300 μm. Such a contact pattern canbe created reliably and reproducibly using conventional screen printingtechniques. Much wider or narrower finger-like contacts are alsopossible, however. Corresponding to the gap between the slots, thefinger-like contacts of a contact structure are spaced about 3 mm apartfrom one another.

One or more antireflection layers of a suitable thickness can then alsobe applied to the passivation layer 6, e.g. further oxide, nitride ortitanium oxide layers.

A solar cell according to the invention produced in this manner meetsall the prerequisites necessary for achieving collection efficiency ofmore than 20%. The demand that the diffusion length be greater for theminority charge carriers than the three-fold thickness of the siliconsubstrate is satisfied by the solar cell according to the invention withinexpensive CZ silicon in which the diffusion length L exceeds thesubstrate thickness d by 1.5 times (where d=60 μm, L≧120 μm). Highsurface quality, expressed by a low surface recombination velocity S,can be simply and reliably achieved by means of passivation layers onboth the front and rear sides. High surface quality of S<1000 cm/s canbe set over the emitter by means of oxide passivation. As regards therear-side quality, a surface recombination velocity of S<100 cm/s isrequired, which can be achieved in the solar cell according to theinvention even without further measures. Requisite shading losses ofless than 4 percent are also exceeded by means of the solar cellaccording to the invention, since it exhibits virtually no shading. Lowrequisite reflection values of <4 percent are obtained by using standardantireflection layers. A high fill factor of at least 80 percent is alsoachieved by the invention.

Another advantage of solar cells with contacts applied only to the rearside is that it is easier to mechanically connect different solar cellsto form a module, because no more lead-ins on the front side arenecessary in order to solder corresponding connections on. Thissimplifies the connecting process and increases procedural reliability.The solar cells according to the invention are therefore fully automatedand can be produced on an industrial scale.

What is claimed is:
 1. A solar cell comprising:a crystalline silicon substrate exhibiting crystallographic (110) planes on a front side and a rear side; a flat doped emitter layer on at least the front side; a plurality of elongated slots aligned parallel to crystallographic (111) planes and extending through an entire thickness of the crystalline silicon substrate; a high doping, corresponding to a conductivity type of the flat doped emitter layer, in the elongated slots; a first contact pattern on the rear side for electrical connection of a bulk material; and a second contact pattern on the rear side for electrical connection of the emitter layer, the second contact pattern overlapping the elongated slots at least in part; wherein the elongated slots are crystallographically etched anistopically from the rear side so that crystallographic (111) planes as delimiting surfaces are exposed in the elongated slots, the elongated slots tapering toward the front side of the solar cell with two walls extending at an angle to a surface of the solar cell.
 2. A solar cell according to claim 1, whereinthe crystalline silicon substrate comprises a tricrystal wafer having three monocrystalline, respectively (110)-oriented regions tilted toward one another, mutual boundary areas of the regions extending radially and forming sectors of the tricrystal wafer, and two of the boundary areas forming first-order twin grain boundaries on crystallographic (111) planes.
 3. A solar cell according to claim 1, whereinthe first and second contact patterns comprise imprinted thick-film contacts.
 4. A solar cell according to claim 1, whereinthe elongated slots are regularly distributed over the surface of the solar cell and have a width of 5 to 50 μm.
 5. A solar cell according to claim 1, whereinthe elongated slots extend parallel to crystallographic (111) planes, but are offset against one another.
 6. A solar cell according to claim 1, whereinthe first and second contact patterns comprise finger-like contacts which interdigitally engage with one another and at least one bus structure which respectively connects all the fingerlike contacts together, one of the bus structures arranged circumferentially on the outside proximate to an edge of the solar cell on the rear side.
 7. A method of producing a solar cell comprising front-side contacts displaced to the rear side, comprisingproviding a (110)-oriented crystalline silicon substrate; etching, from the rear side, in an alkaline, crystal-oriented and masked manner parallel to crystallographic (111) planes into the silicon substrate, a plurality of slots extending through an entire thickness of the silicon substrate; producing by diffusion of a dopant a flat emitter layer; and producing on the rear side, by imprinting and burning-in a conductive paste, a first contact pattern and a second contact pattern, the second contact pattern being disposed to overlap the slots.
 8. A method according to claim 7, whereindiffusion of the dopant takes place all over; doping the emitter layer in the region of the first contact pattern is overcompensated by correspondingly doping the paste during the burning-in process; and the emitter layer on the rear side is separated between first and second contact patterns.
 9. A method according to claim 7, whereinetching the plurality of slots is performed with a photolithographically structured etching mask made of nitride or oxide.
 10. A method according to claim 7, whereinetching the plurality of slots is performed with controlled timing and completed just when an opening which extends through the silicon substrate has been produced.
 11. A method according to claim 7, whereinthe length of the plurality of slots is chosen in accordance with the thickness of the silicon substrate such that the virtual intersection of both crystallographic (111) planes which extend at an angle to a surface of the silicon substrate and delimit each slot is just outside the silicon substrate above the front side.
 12. A method according to claim 7, whereinthe emitter layer on the rear side is separated between the first and second contact patterns by means of masked etching, the first and second contact patterns being used as a mask. 